<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">

<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">

<div>
  结果 <span></span>
</div>

<script>
  function f(x) {
    //获取两个文本框 给元素添加id  浏览器会自动生成以下代码
    //let d1 = document.getElementById("d1");
    //let d2 = document.getElementById("d2");
    let span = document.querySelector("span");
    switch (x){
      case 1:
        span.innerText =i1.value*1+i2.value*1;
        break;
      case 2:
        span.innerText = i1.value-i2.value;
        break;
      case 3:
        span.innerText = i1.value*i2.value;
        break;
      case 4:
        span.innerText = i1.value/i2.value;
        break;
    }
  }

</script>
</body>
</html>